Systems that provide stereophonic and surround-sound effects can enhance consumer experiences in many contexts. In the entertainment industry, for example, stereophonic and surround-sound systems may be used to provide a more realistic feel for movies, video games, and audio tracks. In recent years, researchers have begun to investigate methods for enhancing the audio experience for consumers by attempting to create spatial sound reproduction systems (also called 3D audio, virtual auditory display, virtual auditory space, and virtual acoustic imaging systems) that can make audio playback seem to a consumer as though a given sound originates from a direction, regardless of whether there is actually a speaker situated in the position from which the sound seems to originate. Some of these approaches, such as the wave field synthesis method and the loudspeakers-walls method, use a large number of speakers (e.g., one to three hundred speakers). Others, such as the virtual surround sound method, use sophisticated sound wave modification methods, which may incorporate head-related transfer functions (HRTFs) to simulate spatial sound using a few speakers (e.g., two or three in-line speakers).
A head-related transfer function, which is also sometimes referred to as an external-ear transfer function, is a function that is meant to model the way in which an external ear transforms sounds (i.e., an acoustic signals) heard by a human. The external ear, including the pinna, has transforming effects on sound waves that are ultimately perceived by the eardrum (i.e., the tympanic membrane) in humans. The external ear can, for example, act as a filter that reduces low frequencies, a resonator that enhances middle frequencies, and a directionally dependent filter at high frequencies that assists with spatial perception. Ideally, if an HRTF is accurate, the HRTF can be used by spatial sound reproduction systems to assist in creating the desired illusion that sound originates from a specific direction relative to a user.